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 Post subject: Surface heat transfer coefficients and heat flux when U=0 W/m2K
PostPosted: Wed Apr 29, 2015 7:00 am -1100 
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WUFI User
WUFI User

Joined: Tue Jan 21, 2014 8:43 am -1100
Posts: 8
Hello,
We have a little doubt.
We are trying to simulate the drying process of a material with WUFI-2D. In allways simulation that we had performed until now we considered a surface heat transfer coefficients of 8 or 17-25 W/m2K for indoor or outdoor surfaces respectively.
However, we want to simulate the drying process of a test specimen with an initial conditions of 20ºC and 100% RH in a room with a constant climate of 20 °C and 60% RH over time. Due to the temperature is constant we consider a heat transfer coefficient of 0 in all surfaces but, in this way, the moisture content of the test specimen does not change over time (free water saturation). Thus, it is always necessary to enter a value different from 0 for the surface heat transfer coefficients? In our case, we considered that there is no heat flux because the temperature was constant and we considered U=0 W/m2K but why there is no transfer of moisture? Nevertheless, when we consider a surface heat transfer coefficient of 1,4,8,17 or 25 W/m2K the test speciment dried and the behavior was similar.
It seems that when U=0 W/m2K the surface performed like a adiabatic border without heat and moisture transfer, why?
Thanks,


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 Post subject: Re: Surface heat transfer coefficients and heat flux when U=0 W/m2K
PostPosted: Thu Apr 30, 2015 3:50 am -1100 
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WUFI Administrator
WUFI Administrator

Joined: Sun Jun 19, 2005 10:33 pm -1100
Posts: 242
ITeCons wrote:
It seems that when U=0 W/m2K the surface performed like a adiabatic border without heat and moisture transfer, why?

Hi ITeCons,
yes, setting the heat transfer coefficient of a surface to zero will result in an adiabatic surface. The heat transfer coefficient alpha describes how much heat is flowing through the thin stagnant air layer sticking to the material surface (see the online help: Surface Transfer Coefficients | Heat Transfer Coefficients):

heat flow = alpha * (surface temperature - ambient air temperature)

If alpha is set to zero, the heat flow will always be zero, independent of the temperatures.

The reason why setting alpha to zero also suppresses the vapor flow across the surface is that there is a vapor transfer coefficient beta which describes how much vapor is flowing through the same stagnant air layer at the surface (see the online help: Surface transfer coefficients | Water Vapor Transfer Coefficients):

vapor flow = beta * (surface vapor pressure - ambient vapor pressure).

Since both heat and vapor flow are affected by the same air layer, alpha and beta are (approximately) proportional and WUFI can compute beta from the known alpha. WUFI does this automatically and the user never has to care explicitly about beta. By setting alpha to zero, you also set the beta to zero, eliminating any heat or vapor flow across the material surface.

Please note that in your case there is no reason to set the alpha to zero, anyway. If the specimen is very wet (high moisture content at the surface), the stagnant air layer at the surface will provide the main resistance for the vapor flow across the surface and will thus control the drying rate. Our experience shows that during the initial, very wet drying stage the drying rate of a specimen can be strongly affected by the air speed in the laboratory (there may be a ventilator to ensure constant air conditions throughout the room). This influence of the air speed must then be described by choosing appropriate values for the alpha (and, by implicit consequence, for the beta).

Please note also that, even if the alpha were set to zero and the beta to some nonzero value (allowing vapor flow but no heat flow) the simulation would give unrealistic results. During the drying process, latent heat is absorbed and carried away by the vapor. This heat loss is compensated by heat flowing from the ambient air into the specimen. Allowing the vapor (and with it the latent heat) to escape but preventing heat to flow into the specimen would result in strong and unrealistic cooling of the specimen. So please leave the alpha (and thus the beta) at some value which is appropriate for the air circulation conditions in your laboratory (most likely something close to the typical indoor value of 8 W/m2K).

Kind regards,
Thomas


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